This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Load commutated inverters (LCIs) have proven to be a robust industrial solution to drive large electric motors. For example, in the oil and gas industry, large electric motors are often used in various production processes, such as gas processing and fluid pumping, among others. Load commutated inverters provide an effective way of converting the electrical characteristics of the power received from the power source to the desired electrical characteristics suitable for driving the motor. For example, a load commutated inverter may generate a sinusoidal output current with the desired magnitude and frequency for controlling the speed and power of the motor. However, due to the nature of its operation, the load commutated inverter may also tend to generate undesired electrical signals, known as “harmonics.”
A typical load commutated inverter includes an alternating current (AC) to direct current (DC) rectifier coupled to a power source, an output DC to AC inverter coupled to a load, and a DC link coupling the output of the rectifier to the input of the inverter. The AC/DC and DC/AC power conversions are achieved through the switching of semiconductor devices included in the rectifier and inverter. The switching operation generates a pulsed current waveform at both AC sides. The pulsed current waveform includes the desired fundamental sinusoidal current, together with undesirable harmonic components.
The various harmonics and subharmonics generated by a load commutated inverter may tend to create pulsating torques in the motor-driven machinery to which it is coupled. If the frequency of these pulsating torques is close to the natural resonance frequency of the machinery, the pulsating torque could potentially subject the machinery to torsional damage. Because the natural resonance frequency of machinery is usually relatively low, the low-frequency subharmonics may be more likely to excite a torsional resonance in the machinery.
To mitigate the potential for pulsating torques, various techniques have been developed to suppress harmonic current signals in load commutated inverters. For example, a series inductor is usually disposed on the DC link to filter harmonics. However, the size of the inductor can only be limited. Thus, in practice, the inductor cannot completely remove the subharmonics that are due to the switching of rectifier and inverter at different fundamental frequencies.
Another technique for filtering harmonics is described in a publication by J. J. Simond, et al., entitled “12-Pulse LCI Synchronous Drive for a 20 MW Compressor Modeling, Simulation, and Measurements,” presented at the IEEE IAS Annual Meeting, September 2005. However, the techniques disclosed therein only achieve cancellation of the harmonics at the AC input and output, with no cancellation of harmonics on the DC link.
A technique for cancellation of harmonics on the DC link is presented in a publication by M. Beuermann, et al., entitled “Modular Load Commutated Inverters—A Proven Concept for High Power Applications,” presented at the IAS Annual Meeting, September 2008. The techniques described in this publication seek to reduce harmonics on the DC link by coupling a magnetic filter to the DC links of two load commutated inverters. However, coupling a magnetic filter to the DC link in this way introduces a degree of complexity to the design of the power system.
U.S. Pat. No. 4,823,068 to Delmerico, et al., discloses an induction motor drive that includes source-side and load-side converters with individual control channels for the respective converters. The individual control channels may include a cross tie arrangement between the channels for increased system stability. The source-side converter may also include two series connected bridge rectifiers that may be used to help reduce the harmonics on the source side. However, this arrangement may only cancel harmonics due to the rectifier circuitry on the source side, not on the load side. Therefore, harmonics induced within the current after it has passed through the source side may not be mitigated by the induction motor drive.